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In aqueous solution, precipitation is the process of transforming a dissolved
substance Substance may refer to: * Substance (Jainism), a term in Jain ontology to denote the base or owner of attributes * Chemical substance, a material with a definite chemical composition * Matter, anything that has mass and takes up space * Substance th ...
into an insoluble
solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied ...

solid
from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading to precipitation, the chemical reagent causing the solid to form is called the ''precipitant''. The clear liquid remaining above the precipitated or the centrifuged solid phase is the 'supernate' or 'supernatant'. The notion of precipitation can also be extended to other domains of chemistry (organic chemistry and biochemistry) and even be applied to the solid phases (''e.g.'', metallurgy and alloys) when solid impurities segregate from a solid phase.


Supersaturation

The precipitation of a compound may occur when its concentration exceeds its
solubility In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atom ...

solubility
. This can be due to temperature changes, solvent evaporation, or by mixing solvents. Precipitation occurs more rapidly from a strongly
supersaturated Supersaturation occurs with a chemical solution when the concentration of a solute exceeds the concentration specified by the value equilibrium solubility. Most commonly the term is applied to a solution of a solid in a liquid. A supersaturated sol ...
solution. The formation of a precipitate can be caused by a chemical reaction. When a
barium chloride Barium chloride is an inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemica ...

barium chloride
solution reacts with
sulphuric acid Sulfuric acid ( American spelling) or sulphuric acid ( Commonwealth spelling), also known as oil of vitriol, is a mineral acid composed of the elements sulfur Sulfur (in traditional laity, lay Commonwealth English: sulphur) is a chemic ...

sulphuric acid
, a white precipitate of
barium sulfate Barium sulfate (or sulphate) is the inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter ...

barium sulfate
is formed. When a
potassium iodide Potassium iodide is a chemical compound, Pharmaceutical drug, medication, and dietary supplement. As a medication it is used to treat hyperthyroidism, in radiation emergencies, and to protect the thyroid gland when certain types of radiopharmac ...

potassium iodide
solution reacts with a
lead(II) nitrate Lead(II) nitrate is an inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, prop ...
solution, a yellow precipitate of
lead(II) iodide Lead(II) iodide or lead iodide is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Salt (chemistry), salts; salt in its natural form as a crystallinity, crystalline mi ...
is formed.


Nucleation

An important stage of the precipitation process is the onset of
nucleation Nucleation is the first step in the formation of either a new or a new structure via or . Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appear ...

nucleation
. The creation of a solid
particle In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which can be ascribed several physical property, physical or chemical property, chemical p ...
implies the formation of an
interface Interface or interfacing may refer to: Academic journals * Interface (journal), ''Interface'' (journal), by the Electrochemical Society * ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Lin ...
with the solution. This involves
energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...
changes depending on the dissolution reaction free energy (
endothermic In thermochemistry Thermochemistry is the study of the heat energy which is associated with chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substanc ...
or
exothermic In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these qu ...
process accompanied by an
entropy Entropy is a scientific concept as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamic ...

entropy
increase) and the relative
surface energy Surface free energy or interfacial free energy or surface energy quantifies the disruption of intermolecular bonds that occurs when a surface is created. In the of s, surfaces must be intrinsically less than the bulk of a material (the molecules ...
developed between the solid and the solution. If energy changes are not favorable, or without suitable nucleation sites, no precipitation occurs and the solution remain supersaturated.


Inorganic chemistry


Precipitation in aqueous solution

A common example of precipitation reaction in aqueous solution is that of
silver chloride Silver chloride is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havin ...

silver chloride
. When
silver nitrate Silver nitrate is an inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, be ...

silver nitrate
(AgNO3) is added to a solution of
potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. Covalently bonded metal halides may be discrete m ...

potassium chloride
(KCl) the precipitation of a white solid (AgCl) is observed. :AgNO3 + KCl -> AgCl (v) + KNO3 The ionic equation allows to write this reaction by detailing the
dissociated Dissociation in chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds compos ...
ions An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are u ...

ions
present in aqueous solution. :Ag+ + NO3^- + K+ + Cl^- -> AgCl (v) + K+ + NO3^-


Reductive precipitation

The Walden reductor is an illustration of a reduction reaction directly accompanied by the precipitation of a less soluble compound because of its lower chemical valence: :Cu + 2 Ag+ -> Cu^2+ + 2 Ag The Walden reductor made of tiny
silver Silver is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical ele ...

silver
crystals obtained by the immersion of a
copper Copper is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

copper
wire into a solution of
silver nitrate Silver nitrate is an inorganic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, be ...

silver nitrate
is used to reduce to their lower valence any metallic ion located above the silver couple in the
redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species A chemical species is a chemical substance A chemical substance is a form of matter In clas ...
scale.


Precipitate colors

Many compounds containing
metal A metal (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

metal
ions produce precipitates with distinctive colors. The following are some typical colors for various metals. However, many of these compounds can produce colors very different from those listed. Many compounds often form white precipitates.


Anion/cation qualitative analysis

Precipitate formation is useful in the detection of the type of
cation An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are u ...
in a
salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure fo ...
. To do this, an
alkali In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo dur ...

alkali
first reacts with the unknown salt to produce a precipitate that is the
hydroxide Hydroxide is a diatomic anion with chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other sym ...
of the unknown salt. To identify the cation, the color of the precipitate and its solubility in excess are noted. Similar processes are often used in sequence – for example, a
barium nitrate Barium nitrate is the inorganic compound In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, proper ...

barium nitrate
solution will react with
sulfate The sulfate or sulphate ion is a polyatomic anion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having ...

sulfate
ions to form a solid
barium sulfate Barium sulfate (or sulphate) is the inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter ...

barium sulfate
precipitate, indicating that it is likely that sulfate ions are present.


Colloidal suspensions

Without sufficient attraction forces (''e.g.'',
Van der Waals force In molecular physics Molecular physics is the study of the physical properties of molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings ...
) to aggregate the solid particles together and to remove them from solution by gravity (
settling Settling is the process by which particulates settle to the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to Centrifuge, centrifugal motion will tend to move in a uniform manner in the dir ...
), they remain in
suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in mathematics * Suspension (chemistry), small solid particles suspende ...
and form
colloids A colloid is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, co ...
.
Sedimentation Sedimentation is the deposition of sediments Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks In geology Geology (from the Ancient Greek γ ...
can be accelerated by high speed
centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when v ...

centrifugation
. The compact mass obtained so is sometimes referred to as a 'pellet'.


Digestion and precipitates ageing

Digestion, or ''precipitate ageing'', happens when a freshly formed precipitate is left, usually at a higher
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
, in the solution from which it precipitates. It results in purer and larger recrystallized particles. The physico-chemical process underlying digestion is called
Ostwald ripening Ostwald ripening is a phenomenon observed in solid solutions or liquid sols that describes the change of an inhomogeneous structure over time, i.e., small crystals or sol particles dissolve, and redeposit onto larger crystals or sol particles. ...
.


Organic chemistry

While precipitation reactions can be used for making
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compound , CH4; is among the simplest organic compou ...
s, removing ions from solution in
water treatment Water treatment is any process that improves the quality Quality may refer to: Concepts *Quality (business), the ''non-inferiority'' or ''superiority'' of something *Quality (philosophy), an attribute or a property *Quality (physics), in respo ...

water treatment
, and in classical qualitative inorganic analysis, precipitation is also commonly used to isolate the products of an organic reaction during workup and purification operations. Ideally, the product of the reaction is insoluble in the solvent used for the reaction. Thus, it precipitates as it is formed, preferably
forming pure crystals
forming pure crystals
. An example of this would be the synthesis of
porphyrin Porphyrins ( ) are a group of heterocyclic 125px, Pyridine, a heterocyclic compound A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different chemical element, elements as members of its ring(s). ...

porphyrin
s in refluxing
propionic acid Propionic acid (, from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population ...

propionic acid
. By cooling the reaction mixture to room temperature, crystals of the porphyrin precipitate, and are collected by filtration on a Büchner filter as illustrated by the photograph here beside: Precipitation may also occur when an ''antisolvent'' (a solvent in which the product is insoluble) is added, drastically reducing the solubility of the desired product. Thereafter, the precipitate may be easily separated by
decanting Decantation is a process for the separation of mixtures of immiscible Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration In chemistry Chemistry is ...
,
filtration Filtration is a physical separation process A separation process is a method that converts a mixture or solution of chemical substances into two or more distinct product mixtures. At least one of results of the separation is enriched in one or m ...
, or by
centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when v ...
. An example would be the synthesis of Cr3+ tetraphenylporphyrin chloride: water is added to the
dimethylformamide (DMF)
dimethylformamide (DMF)
solution in which the reaction occurred, and the product precipitates. Precipitation is useful in purifying many other products: ''e.g.'', crude
bmim 1-Butyl-3-methylimidazolium hexafluorophosphate, also known as BMIM-PF6, is a viscous, colourless, hydrophobe, hydrophobic and non-water-soluble ionic liquid with a melting point of -8 °C. Together with 1-butyl-3-methylimidazolium tetrafluor ...

bmim
-Cl is taken up in
acetonitrile Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chem ...

acetonitrile
, and dropped into
ethyl acetate Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), million ...

ethyl acetate
, where it precipitates.


Biochemistry

Protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

Protein
s purification and separation can be performed by precipitation in changing the nature of the solvent or the value of its
dielectric constant The dielectric constant (or relative permittivity) is the permittivity, electric permeability of a material expressed as a ratio with the vacuum permittivity, electric permeability of a vacuum. A dielectric is an insulating material, and the diele ...
(''e.g.'', by replacing water by
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
), or by increasing the
ionic strength The ionic strength of a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute and the ...
of the solution. As proteins have complex tertiary and quaternary structures due to their specific folding and various weak intermolecular interactions (''e.g.'', hydrogen bridges), these superstructures can be modified and proteins denaturated and precipitated. Another important application of an antisolvent is in ethanol precipitation of
DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically neutral gro ...

DNA
.


Metallurgy and alloys

In solid phases, precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, due to e.g. rapid quenching or
ion implantation technological facility in Toulouse, France. Ion implantation is a low-temperature process by which ion (physics), ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the t ...
, and the temperature is high enough that diffusion can lead to segregation into precipitates. Precipitation in solids is routinely used to synthesize
nanoclusters
nanoclusters
. In
metallurgy Metallurgy is a domain of materials science and engineering ''Materials Science and Engineering'' may refer to several journals in the field of materials science and engineering: * '' Materials Science and Engineering A'' * '' Materials Science ...
, precipitation from a
solid solution A solid solution describes a family of materials which have a range of compositions (e.g. AxB1−x) and a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solution" is used to desc ...

solid solution
is also a way to strengthen alloys. Precipitation of
ceramic phases
ceramic phases
in metallic alloys such as
zirconium hydride Zirconium hydride describes an alloy An alloy is an admixture of metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a ...
s in
zircaloy Zirconium alloys are solid solutions of zirconium or other metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous ap ...
cladding of
nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of Fissile mate ...
pins can also render metallic alloys brittle and lead to their mechanical failure. Correctly mastering the precise temperature and pressure conditions when cooling down
spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...
s is therefore essential to avoid damaging their cladding and to preserve the integrity of the spent fuel elements on the long term in dry storage casks and in geological disposal conditions.


Industrial processes

Hydroxide precipitation is probably the most widely used industrial precipitation process in which metal hydroxides are formed by adding
calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula calcium, Ca(Hydroxide, OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaking (ge ...

calcium hydroxide
(''slaked lime'') or
sodium hydroxide Sodium hydroxide, also known as lye A lye is a metal hydroxide traditionally obtained by leaching wood ashes, or a strong alkali In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chem ...

sodium hydroxide
(''caustic'' ''soda'') as precipitant.


History

Powders derived from different precipitation processes have also
historically History (from Ancient Greek, Greek , ''historia'', meaning "inquiry; knowledge acquired by investigation") is the study of the past. Events occurring before the History of writing#Inventions of writing, invention of writing systems are considered ...
been known as 'flowers'.


See also

*
CoprecipitationIn chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they undergo ...
*
Effervescence Image:Soda bubbles macro.jpg, Bubbles of carbon dioxide float to the surface of a Soft drink#Carbonated drinks, carbonated soft drink. Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from that rel ...
, the "up-arrow" *
Salting in Salting in refers to the effect where increasing the ionic strength The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic Chemical compound, compounds, when solubility, dissolved in water, Dissociation (c ...
*
Salting out Salting out (also known as salt-induced precipitation, salt fractionation, anti-solvent crystallization, precipitation crystallization, or drowning out) is a purification technique that utilizes the reduced solubility of certain molecules in a sol ...


References


Additional reading

*


External links


Precipitation reactions of certain cations

Digestion Instruments

A Thesis on pattern formation in precipitation reactions
{{DEFAULTSORT:Precipitation (Chemistry) Liquid-solid separation